The discharge of acid plume, especially visible acid plume, is caused by sulfur trioxide (SO3), which in turn is caused by the oxidation of sulfur dioxide (SO2). The problem is especially troublesome for coal-fired combustors burning coals having high sulfur contents and is exacerbated when the coal contains high levels of iron, which is involved in corrosion and can result in an increased production of SO3.
The formation of sulfuric acid in boilers typically occurs when sulfur in a fuel is oxidized. Almost all of the sulfur is oxidized to SO2 in the combustion zone because, at combustion temperatures, SO2 is the equilibrium form of oxidized sulfur. As the gas temperature falls between the combustion zone and the stack plume, equilibria shifts occur. While SO2 is the most stable species in the combustion zone, a small percent of SO2 in the gas, e.g., from about ½ to about 2%, converts to SO3 as temperatures drop below 1,400° F. SO3 levels increase across the selective catalytic reduction chamber (SCR), if present, and concentrations remain stable in the gas. At around 800° F. some of the SO3 converts to sulfuric acid (H2SO4) in the presence of water vapor in the gas. This shift of SO3 to H2SO4 in the combustion gas continues until almost all of the SO3 is converted to H2SO4 by 600 degrees F. The rest of the SO3 is converted by the time the gas exits air preheater. Additional kinetic factors influencing oxidation of SO2 include the fuel sulfur content, excess combustion air levels and catalytic oxidation associated with some metals in the ash and on the boiler tube surfaces, especially iron and vanadium.
The use of catalytic units to reduce NOx in the combustion gases have also been seen to increase SO3 because the catalyst employed to reduce NOx with a nitrogenous reducing agent has the tendency to oxidize SO2 to SO3. This oxidation tendency is thought to increase as the NOx reducing capability is lost as the catalyst degrades due to the normal aging process.
Some, but not all, coal-fired combustors will include a wet flue gas desulfurizing unit (FGD). Wet FGD systems tend to reduce SO3 levels, but they also typically convert SO3 to sulfuric acid (H2SO4) mist. While there is no general rule, SO3 reduction can be in the range of about 30% to about 40%; however, scrubbers are not effective at removing sulfuric acid aerosols, and most of the sulfuric acid mist slips past the scrubber and exits the stack, often visible as a blue plume.
While the art has proposed many solutions to the problem of plume, none have been fully effective 100% of the time. Among the approaches that have been considered are: alkali injection into the furnace, humidification at an ESP inlet to reduce the temperature to below the acid dew point, alkali injection combined with humidification at the ESP inlet, a separate wet particulate control device such as a wet ESP, alkali injection in ducting leading to a wet FGD scrubber and an electrostatically augmented mist eliminator. However, sometimes load fluctuations cause SO3 problems beyond the ability of a particular technology to meet increased acid plume and anomalies occur that have not been fully addressed.
In one particular scenario, a serious problem relates to burning a high-sulfur coal, also having a high iron content, in a combustor outfitted with a selective catalytic reduction unit for NOx control and utilizing magnesium hydroxide injection into the furnace for control of slag, corrosion and fouling. Due to the chemistry involved, the magnesium hydroxide injected in this manner has been successful in controlling blue plume as well as slag, corrosion and fouling under most circumstances; however, when load is shifted up and down, the visible blue plume cannot be fully handled at all times.
Whether or not an FGD is employed, visible blue plume is a continuing problem and is a consequence of SO3 concentrations in the stack gas emissions being over 5 to 15 ppm.
There is a need to better control visible blue plume, especially under conditions of fluctuating load.